1) Field of the Invention
This invention relates to an optical waveguide device and an optical modulator for use with optical communication.
2) Description of the Related Art
In recent years, expectations on an optical modulator of the external modulation type (external modulator) have been and are increasing in order to realize a very high-speed and broadband optical communication network system.
Particularly, in order to realize long haul transmission of a light signal, a Mach-Zehnder type (MZ) optical modulator (MZ type LN optical modulator) for which LiNbO3 (lithium niobate; LN) is used and which is excellent in a high-speed modulation characteristic and a dispersion resistance characteristic in a broadband has been and is being developed.
In the MZ type LN optical modulator, an operating point is fluctuated by a temperature drift, a DC drift or the like, and therefore, a bias voltage is applied in order to compensate for the fluctuation of the operating point. Generally, a monitoring PD (photo-detector, light detection section) is provided on the output side of an optical modulator to detect radiation mode light radiated from a branching portion of a Y-branch optical waveguide on the output side of the MZ type optical waveguide as monitor light, and feedback control for controlling the bias voltage based on the detected monitor light is performed.
However, where the radiation mode light is used as the monitor light as described above, since the intensity of the radiation mode light is low, it is necessary to use a high-sensitivity PD as the monitoring PD for use for detection of the light intensity. Therefore, the degree of freedom in selection of a monitoring PD is low. Also a process of a signal detected by the monitoring PD is limited.
It is a possible idea to provide, for example, as shown in FIG. 18, a 3 dB directional coupler 111 on the output side of an MZ type optical waveguide 110, connect a monitoring optical waveguide 112 to one of ports on the output side of the 3 dB directional coupler 111 to detect the intensity of the monitor light guided through the monitoring optical waveguide 112 and use the detected intensity for the feed back control of the bias voltage.
However, if the monitor light is extracted by such a method as described above, then the intensity of the monitor light is equal to the intensity of output light (signal light) as seen in FIG. 19. Consequently, there is the possibility that, if the intensity of the input light is high, then the intensity of the input light may be so high as the intensity of monitor light that the monitoring PD may be broken.
Incidentally, an RZ (Return to Zero) optical modulator which performs modulation with a clock signal and modulation with a data signal using two stages of Mach-Zehnder type optical modulators coupled in series to produce an RZ signal has been proposed recently.
In such an RZ optical modulator as described above, it is a possible idea to detect, for example, as shown in FIGS. 20(A) and 20(B), only radiation mode light radiated from a branching portion of a Y-branch optical waveguide 115 on the output side of an Mach-Zehnder type optical modulator 114 provided on the latter stage from within two stages of Mach-Zehnder type optical modulators 113 and 114 as monitor light by means of a monitor PD 116 and then perform bias control of both of the two-stages of the Mach-Zehnder type optical modulator 113 and 114 based on the detected light.
However, in such a method as described above, it is difficult to perform accurate bias control, and further, the control is complicated.